Hopefully this doesn't violate any forum rules. I have NO commercial links with this company.
I often see on various fora questions about renting motorcycles for a day or so during a trip to Japan. At last week's TMCS, I saw a booth for Rental 819 (which kind of spells out "bike" in Japanese), so I picked up their English pamphlet in order to post it online for interested folk.
The basic requirements are:
1. Be at least 20 years old.
2. Valid "home" motorcycle license
3. International license
4. Passport
The website is http://www.rental819.com
Contact edress is: info@rental819.com
Scanned price list:

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You can only be young once, but you can be immature forever!

Well it's pricey for sure, and I would guess a daunting idea for the uninitiated, but Japan is a real hidden gem and has some of the best riding roads the world has to offer. Outside of the crowded cities, the country is like 80% mountainous (NC, WV type mountains) - one only has to google map to see how twisty some of these roads are.

Thanks for posting this, I need to make back out there sometime for a real adventure..

(btw, it's been many years since I've ridden there, but one of the best resources I can recall was a quarterly bike magazine whose purpose was to map out the best riding roads.... does it still exist?)

I think one of the other requirements is to have a fluent Japanese speaker with you when you rent the bike. But that might not be a problem for you.
I want to rent something to ride around Tokyo when I visit in May, but my friends don't have licenses.

(btw, it's been many years since I've ridden there, but one of the best resources I can recall was a quarterly bike magazine whose purpose was to map out the best riding roads.... does it still exist?)

I don't know offhand which specific magazine you're referring to, but there are several such periodicals out here. My favorite is the yearly updated Touring Mapple R, a motorcycle specific map series which comes out in separate issues to cover different regions of the country, and is sized to fit a tank bag window. The maps also provide a lot of info for dual purpose bikes, such as the condtions of various ungraded roads, season specific road closings, etc. Not that I'm into DP bikes myself, but I know of LOT of the folks here on ADV are.

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You can only be young once, but you can be immature forever!

Good to see they still have mags like that... I have no idea what I was looking at (it's been decades) but I do remember finding this magazine, following a couple routes, and riding the best roads I've ever been on in my life (and I've been to NC snd WV a few times).